Method for exfoliating carbonization catalyst from graphene sheet, method for transferring graphene sheet from which carbonization catalyst is exfoliated to device, graphene sheet and device using the graphene sheet

ABSTRACT

A carbonization catalyst for forming graphene may be exfoliated from a graphene sheet by etching. A binder layer may be formed on the graphene sheet on which a carbonization catalyst is formed, to support and fix all or part of the graphene sheet. Further, the graphene sheet from which the carbonization catalyst is exfoliated may be transferred to a device. When exfoliating the carbonization catalyst from the graphene sheet, an acid may be used together with a wetting agent.

This application claims priority to Korean Patent Application No.10-2008-0055310, filed on Jun. 12, 2008, and all the benefits accruingtherefrom under U.S.C. §119, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND

1. Field

This disclosure relates to a method for exfoliating a carbonizationcatalyst from a graphene sheet grown on the carbonization catalyst, amethod for transferring the graphene sheet to a device, a graphene sheetfrom which a carbonization catalyst is exfoliated and a devicecomprising the graphene sheet.

2. Description of the Related Art

Graphite may be formed of a stacked structure of two-dimensional planarsheets in which carbon atoms may be bonded in an extended fused array ofhexagonal rings. A single sheet of the extended fused array ofsix-membered carbon rings may be referred to as graphene.

Graphene sheet, as defined herein, may comprise one or more sheets ofgraphene. Graphene sheet may have advantageous properties different fromthose of other materials. In particular, electrons may move on thegraphene sheet as if they have zero mass, which means that the electronsmay move at the velocity at which light moves in vacuum. Electronmobility on graphene sheet has been found to be about 20,000 to 50,000cm²/Vs. Further, graphene sheet may exhibit unusual half-integer quantumhall effects for electrons and holes.

Since the electrical properties of graphene sheet with a given thicknessmay be changed depending on its crystal orientation, the electricalproperties may be controlled by selecting the crystalline orientation ofthe graphene sheet and to this end, designing devices with differentelectrical properties using graphene sheet may be relativelystraightforward. The electrical properties of graphene sheet may becompared with those of a carbon nanotube (“CNT”), which is known toexhibit metallic or semiconducting properties dependent upon thechirality and diameter of the CNT. A complicated separation process maybe required in order to take advantage of such metallic orsemiconducting properties of CNTs. Graphene sheet may also have economicadvantages over CNTs in that because no purification may be needed, aswith synthesized CNTs, graphene sheets may be less expensive. Therefore,graphene sheet may be widely used for carbon-based electrical orelectronic devices.

Graphene sheets may be prepared generally by a micromechanical processor by a SiC crystal pyrolysis process.

A micromechanical process may be a method that may include, for example,attaching a tape onto a surface of a graphite sample, and releasing thetape from the surface by peeling, to obtain a graphene sheet adhered tothe tape coming off the graphite. The tape may be then released from thegraphene sheet by, for example, dissolving the tape in a solvent.

The SiC crystal pyrolysis process may be a method that may include, forexample, heating a SiC single crystal to decompose SiC on the surface ofthe crystal. The Si may be removed after the decomposition, and theremaining carbon (C) may form the graphene sheet.

SUMMARY

A variety of novel techniques for manufacturing graphene sheets may beimplemented by forming graphene on a carbonization catalyst using carbonsource. Large-sized graphene sheets may be reproduced economically usingthese techniques.

In these techniques, graphene sheet may be easily damaged whenexfoliating the graphene sheet from the carbonization catalyst after thegraphene sheet is formed on the carbonization catalyst. Forming a binderlayer on the graphene sheet may be used for preventing the damage of thegraphene sheet which may occur when the carbonization catalyst isexfoliated from the graphene sheet.

Disclosed herein is, in an embodiment, a method for exfoliating acarbonization catalyst from a graphene sheet wherein the method mayinclude forming a binder layer on the graphene sheet, wherein the binderlayer may support and fix all or part of the graphene sheet formed onthe carbonization catalyst, and exfoliating the carbonization catalystfrom the graphene sheet.

Also in an embodiment, a method for transferring the graphene sheet fromwhich a carbonization catalyst is exfoliated to a device may includeforming a binder layer on a surface of a graphene sheet formed on asurface of a carbonization catalyst where the binder layer andcarbonization catalysts may be on opposite surfaces of the graphenesheet, wherein the binder layer may support and fix all or part of thegraphene sheet, exfoliating the carbonization catalyst from the graphenesheet, and transferring the graphene sheet from which the carbonizationcatalyst is exfoliated to a device.

In another embodiment, a graphene sheet may be obtained by forming abinder layer on a surface of the graphene sheet which is formed on asurface of a carbonization catalyst, where the binder layer andcarbonization catalysts may be on opposite surfaces of the graphenesheet and the binder layer may support and fix all or part of thegraphene sheet formed on the carbonization catalyst, and exfoliating thecarbonization catalyst from the graphene sheet.

In another embodiment, a method of forming a device, may include forminga graphene sheet on a surface of a carbonization catalyst, forming abinder layer on a surface of the graphene sheet opposite thecarbonization catalyst, wherein the binder layer supports and fixes allor part of the graphene sheet formed on the carbonization catalyst,exfoliating the carbonization catalyst from the graphene sheet, andtransferring the graphene sheet to the device. In a further embodiment,a device may include the graphene sheet from which the carbonizationcatalyst is exfoliated by the aforesaid method.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the disclosed embodimentswill be more apparent from the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic view of a carbonization catalyst formed to have afilm shape and a graphene sheet, according to an exemplary embodiment;

FIGS. 2 and 3 are schematic views of a binder layer formed on a graphenesheet, according to an exemplary embodiment;

FIG. 4 is a schematic view of a wet etching process for exfoliating acarbonization catalyst, according to an exemplary embodiment;

FIG. 5 is a schematic view showing a wetting agent further added to theacid etching solution in FIG. 4;

FIG. 6 is a schematic view of the graphene sheet of FIG. 5 removed fromthe etching solution; and

FIG. 7 is a schematic view of a process of transferring the graphenesheet of FIG. 6 to a device.

DETAILED DESCRIPTION

The embodiments will be described more fully hereinafter with referenceto the accompanying drawings, in which exemplary embodiments are shown.It will be appreciated that the invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. In the description, details of well-known featuresand techniques may be omitted to avoid unnecessarily obscuring thepresented exemplary embodiments.

The terminology used herein is for the purpose of describing particularexemplary embodiments only and is not intended to be limiting of theinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. The use of the terms “first”, “second”, and thelike do not imply any particular order, but are included to identifyindividual elements. It will be further understood that the terms“comprises” and/or “comprising”, or “includes” and/or “including” whenused in this specification, specify the presence of stated features,regions, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

In the drawings, like reference numerals in the drawings denote likeelements and shape, size and regions, and the like, are exaggerated forclarity.

FIG. 1 is a schematic view of a carbonization catalyst formed to have afilm shape (referred to “carbonization catalyst film”) and a graphenesheet according to an exemplary embodiment.

Referring to FIG. 1, graphene may be grown on a surface of acarbonization catalyst film 11 to form a graphene sheet 13.

In an embodiment, the carbonization catalyst may be, for example, atleast one selected from the group consisting of Ni, Co, Fe, Pt, Au, Al,Cr, Cu, Mg, Mn, Mo, Rh, Si, Ta, Ti, W, U, V, Zr, and any combinationthereof. And, the carbonization catalyst film may be either thin orthick where, when the carbonization catalyst film may be thin, it mayhave a thickness of about 1 to about 1,000 nm, or where thecarbonization catalyst film may be thick, it may have a thickness ofabout 0.01 to about 5 mm.

Graphene may be formed on the surface of the carbonization catalyst byvarious methods. As for a non-limiting example of the methods, achemical vapor deposition (“CVD”) method of supplying a gaseous carbonsource in the presence of a carbonization catalyst and carrying outheat-treatment to grow graphene on the carbonization catalyst may beused. In an embodiment, a carbonization catalyst may be formed to have afilm shape and placed in a chamber without oxygen. Then, heat-treatmentmay be carried out at a temperature of, for example, about 300 to about2,000° C. for about 1 second to about 1 hour while supplying a gaseouscarbon source such as carbon monoxide, ethane, ethylene, ethanol,acetylene, propane, butane, butadiene, pentane, pentene,cyclopentadiene, hexane, cyclohexane, benzene, toluene, combinationsthereof, or the like, at a flow of about 5 to about 1,000 standard cubiccentimeters per minute (sccm), specifically about 10 to about 500 sccm,optionally in the presence of an inert gas such as nitrogen, helium,argon, or the like so as to form graphene. The carbon atoms of thecarbon source may be bonded with each other to form stable fused planarhexagonal shapes with extended pi-electron systems, to produce thegraphene sheet. A graphene sheet with a regular lattice structure may beobtained by cooling the produced graphene sheet (As for a non-limitingexample, the cooling may include a natural cooling and the cooling ratemay be 10˜500° C. per minute). Though a CVD method was described above,methods of growing graphene on the carbonization catalyst may not belimited only to the CVD method.

In another exemplary embodiment, a carbonization catalyst may becontacted with a carbon source such as a liquid carbon-based material.Through a preliminary heat treating of the liquid carbon-based material,the liquid carbon-based material may be decomposed by the carbonizationcatalyst to liberate carbon. The carbon may be implanted into thecarbonization catalyst, which is so called as carburized, and thus,graphene sheet may be formed on the carbonization catalyst layer.Non-limiting examples of the process for contacting the carbonizationcatalyst with the liquid carbon-based material may include immersing,etc. As for a non-limiting example of the liquid carbon-based material,an organic solution may be used. The liquid carbon-based material,however, is not limited to the organic solution and may include anyliquid carbon-based material which may include carbon and be decomposedby the carbonization catalyst. As for non-limiting examples of theorganic solution, polar or non-polar organic solutions having a boilingtemperature of about 60 to about 400° C. may be used. As fornon-limiting examples of the organic solution, alcohol-based organicsolution, ether-based organic solution, ketone-based solution,ester-based organic solution, organic acid-based organic solution, etc.may be used. In terms of a reduction ability, a reactivity, anadsorption with carbonization metal catalyst, alcohol-based organicsolution or ether-based organic solution may be used. In the preliminaryheat treating, the liquid carbon-based material and the carbonizationcatalyst may be stirred to be sufficiently mixed with each other. As fora non-limiting example, the heat treating may be carried out at about100 to about 400° C. for about 10 minutes to about 48 hours.

In another exemplary embodiment, a carbonization catalyst may becontacted with a carbon source such as a carbon-containing polymer inorder to form a graphene sheet. The carbon-containing polymer is notlimited to a specific carbon-containing polymer. As for a non-limitingexample, self-assembly polymer may be used. Self-assembly polymer may beperpendicularly aligned in order on the carbonization polymer to form aself-assembly layer, which may help a high-dense graphene sheet made. Asfor a non-limiting example of the self-assembly polymer, at least oneselected from the group consisting of amphiphilic polymer liquid crystalpolymer, conductive polymer and any combination thereof may be used.

In other exemplary embodiments, any method for forming graphene oncarbonization catalyst may be used as long as graphene is formed oncarbonization catalyst. In this way, those skilled in the art willunderstand that the exemplary embodiments are not limited to aparticular method of forming graphene on carbonization catalyst.

In an exemplary embodiment, the graphene sheet 13 refers to a sheet madeof graphene. As for a non-limiting example of the graphene sheet, thegraphene sheet may be formed of fused polycyclic aromatic rings withcarbon atoms covalently bonded (normally sp²-bonded). The covalentlybonded carbon atoms may form 6-membered ring as a basic repeating unit,but 5- and/or 7-membered rings may further be formed. The graphene sheetmay be a single layer of graphene or may comprise a multi-layered (up toabout 300 layers) graphene. The graphene sheet may include variousstructures. The structure may be varied depending on the content of the5- and/or 7-membered rings included in the graphene. In general, theside ends (i.e., the edges) of the graphene sheet may be saturated withhydrogen atoms.

In an exemplary embodiment, the graphene sheet 13 may be one having alarge area with the length greater than or equal to about 1 mm,specifically about 1 mm to about 1,000 mm along the transverse andlongitudinal directions. Further, the graphene sheet 13 may desirablyhave a homogeneous structure with few defects.

FIGS. 2 and 3 are schematic views of a binder layer formed on a surfaceof a graphene sheet according to an exemplary embodiment. A binder layer15 may be formed on a surface of the graphene sheet 13 on the surfaceopposite the carbonization catalyst film 11 (see FIG. 2). On a surfaceof the binder layer 15 opposite the graphene sheet 13, a substrate 17may further be formed, if necessary (see FIG. 3). As for non-limitingexamples of the substrate, a plastic substrate such as a PET substratemay be used. As for non-limiting examples of the method forming thesubstrate, the substrate may be formed on the binder material before thebinder material coated on the graphene sheet is cured. Further, afterthe binder material is coated on the graphene sheet, the substrate maybe further disposed on the surface of the binder layer opposite thegraphene sheet before the binder material is cured.

Though the binder layer 15 may be formed to cover the entire surface ofthe graphene sheet 13 as in FIGS. 2 and 3, the binder layer may also beformed to cover only a portion of the graphene sheet 13.

Hereinafter is given a detailed description of the binder layer 15.

The graphene sheet may be transferred to a device in order to use thegraphene sheet in the device. It may be necessary to exfoliate thecarbonization catalyst film 11 on which graphene has grown. Thecarbonization catalyst film 11 on which graphene has grown may beexfoliated by wet etching or dry etching. In an embodiment, for example,where wet etching is used, the carbonization catalyst film 11 may beexfoliated by reaction with an acid, which acts as an etchant. In theprocess of exfoliating the carbonization catalyst film 11, there may bea risk that the graphene sheet 13 formed on the carbonization catalystfilm 11 may be damaged. The graphene sheet 13 formed on thecarbonization catalyst film 11 may also be vulnerable to externalvibration or chemical substances, where such vulnerability may increaseif the graphene sheet 13 is thinner.

Thus, in order to prevent possible damage of the graphene sheet 13during the exfoliation of the carbonization catalyst film 11, the binderlayer 15, which may support and fix the graphene sheet 13, may be formedto cover all or part of surface of the graphene sheet 13. The binderlayer 15 may thus serve to not only support the graphene sheet 13 butalso to fix the graphene sheet 13, i.e., to bind the graphene sheet 13.Since the graphene sheet may be fixed, rolling up of the graphene sheet,which may occur when the graphene sheet is exposed to a solvent, may beprevented. For example, when the graphene sheet is exposed to a solvent,the graphene sheet may have a difference in polarity with the solvent(e.g., where the graphene is nonpolar and the solvent is a polarsolvent) so that the graphene sheet may roll up when changed from oneenvironment to another where the graphene interacts with itself inpreference to the solvent, and forms a rolled structure as a more stablestructure. In an embodiment, the graphene sheet may not be rolled uponexposure to air after exfoliating.

As for non-limiting examples of the binder material that may be used forthe binder layer 15, an insulating binder material such as asiloxane-based compound, an acryl-based compound, an epoxy-basedcompound, and the like may be used. Further, a conductive polymer, apolymer electrolyte material, a photoresist (“PR”) material, a metalpaste, and the like may be used as the binder material.

Non-limiting examples of the siloxane-based compound may include, forexample, polydimethylsiloxanes (“PDMS”), polydiphenylsiloxanes (“PDPS”),polysilsesquioxanes (“PSQ”), copolymers thereof, combinations thereof,and the like.

Non-limiting examples of the acryl-based compound may includepoly(methyl methacrylate) (“PMMA”), poly(ethyl methacrylate) (“PEMA”),poly(butyl methacrylate) (“PBMA”), poly(isobutyl methacrylate)(“PIBMA”), copolymers thereof, combinations thereof, and the like.

Non-limiting examples of the epoxy-based compound may include epoxyresin, and the like. For reference, epoxy resin may be produced fromcondensation polymerization of bisphenol A and epichlorohydrin, andvarious epoxy resins may be obtained depending on the proportion of thetwo monomers and/or the molecular weight of the polymer produced.

Non-limiting examples of the conductive polymer may includepolyacetylene, polypyrrole, polyaniline, polythiophene, copolymersthereof, combinations thereof, and the like.

Non-limiting examples of the polymer electrolyte material may includepolyphosphagen, and the like.

Non-limiting examples of the photoresist material may include variousphotosensitive polymers such as cinnamic acid polyvinyl ester, and thelike. Commercial photoresist materials may be used, such as for example,AZ 111 (available from MicroChem).

Non-limiting examples of a metal paste may include Ag paste, and thelike.

Because the carbonization catalyst film 11 may be exfoliated by anetchant (e.g., an acid) as described above, the binder layer 15 per semay also be exposed to the etchant. In order for the binder layer 15 tofully serve as a binder of the graphene sheet, the binder layer 15 may,in some embodiments, be chemically resistant to the etchant, and inother embodiments, may be controllably etched (i.e., damaged) by theetchant. Where controlled etching of the binder is desired, it may beavailable to control the damage of the binder layer 15 due to theetchant (i.e., the dissociation and/or discoloration of the polymer inthe binder layer due to the acid) to be about 50% of the thickness ofthe binder layer 15 or less. Further, it may be available to control thedamage of the binder layer 15 due to the etchant to be about 0% to about10% of the thickness of the binder layer. For example, when the polymerin the binder layer dissociates (i.e, reacts with and/or dissolves) intothe etchant, the etch rate of the carbonization catalyst film 11 maydecrease.

The etchant damage may be evaluated by various methods. As for anon-limiting example, the etchant damage may be evaluated as thepercentage ratio of time required for etching where the binder layer 15is used, to the time required for etching where the binder layer 15 isnot used. For example, for purposes of explanation, an etch process maytake about 10 hours for etching when the binder layer 15 is not used.Similarly, also for purposes of explanation, in two exemplary instancesa binder layer 15 is used, where the etching times required for etchingare respectively about 18 hours and about 11 hours. In this instance,the damage of the binder layer of the two exemplary cases may beevaluated to be about 80% [((18 h.−10 h.)/10 h.)×100] and about 10%[((11 h.−10 h.)/10 h.)×100], respectively. If the damage of the binderlayer may be about 0%, it may further mean that there may be nodiscoloration in addition to the fact that the etching time is the sameas when the binder layer is not used.

For example, when the siloxane-based compound or the acryl-basedcompound is used as the binder material, the etchant damage may be about0%. When the epoxy-based compound, the conductive polymer, thephotoresist material or the metal paste is used as the binder material,the etchant damage may be less than or equal to about 10%. And, when thepolymer electrolyte material is used as the binder material, the etchantdamage may be less than or equal to about 80%.

It may be possible for the binder layer 15 to sufficiently support thegraphene sheet 13 during the exfoliation of the carbonization catalystfilm 11. For the purpose, it may be possible that the binder layer 15may be in a cured state. Therefore, a curable material (e.g., a materialthat may be cured by heat, UV, and the like) may be used for the binderlayer 15. For example, the siloxane-based compound, the acryl-basedcompound, the conductive polymer, the photoresist material, and thelike, may each be curable materials.

After the carbonization catalyst film 11 is exfoliated, the binder layer15 may be either removed or not removed from the graphene sheet 13 to beapplied to a device.

Removal of the binder layer 15, where desired, may be carried out usinga solvent (e.g., an organic solvent where non-limiting examples of theorganic solvent may include ketone-based or alcohol-based organicsolvent). Thus, where the binder layer 15 is to be removed, the binderlayer 15 may be formed of a material that may dissociate into (i.e.,dissolve in) the solvent for easier removal of the binder layer 15. Forexample, the acryl-based compound or the photoresist material may bedissociable in this way (i.e., soluble). In contrast, the siloxane-basedcompound, the epoxy-based compound, the conductive polymer or thepolymer electrolyte material may not be dissociable (i.e., insoluble),as such materials may be thermosetting and therefore solvent resistantafter cure.

Where the binder layer 15 is not removed from the graphene sheet 13, thebinder layer 15 may need to be transparent depending on the device type.For the purpose, the binder layer 15 may be formed of a transparentmaterial. For example, a binder layer made of the siloxane-basedcompound may exhibit a transparency of up to about 100% (i.e., having upto about 100% of incident light transmittance through the binder layer).Binder layer made of the acryl-based compound, the epoxy-based compoundand the polymer electrolyte material may exhibit a transparency ofgreater than or equal to about 90%, greater than or equal to about 80%,and greater than or equal to about 70%, respectively. In contrast, abinder layer made of the conductive polymer may be colored, which thoughdependent on the thickness of the binder layer, may exhibit atransparency of less than about 50% in a given specific region of thevisible light spectrum.

The binder layer 15 may have a thickness of greater than or equal toabout 1,000 Å. Reduction of the thickness of the binder layer 15 to beless than about 1,000 Å may become increasingly difficult. Thought theremay be no particular requirement for an upper limit for the thickness ofthe binder layer 15, as for a non-limiting example, the binder layer maybe 1000 Å-10 μm. For another non-limiting example, the binder layer maybe 5000 Å-1 μm. The concrete thickness may be varied according to theconcrete applications. If the binder layer 15 is formed to support andfix a part of graphene sheet 13, the part may be chosen in order for thegraphene sheet to be prevented from rolling when the graphene sheet isexposed to a solvent. As for non-limiting examples such as an example 7of below described experiment 1, if the binder material is coated tooverlap the edge of the nickel plate and the graphene sheet in order forthe binder layer to support and fix the graphene sheet at its edgeportion, the binder layer may be formed to have an area of 4%-16% oftotal graphene sheet area (the same as the area of the catalyst layer)at the edge portion of graphene sheet or to have an area of less than20% of total graphene sheet area (the same as the area of the catalystarea) at the periphery of the graphene sheet.

FIG. 4 is a schematic view of a wet etching process for exfoliating acarbonization catalyst according to an exemplary embodiment.

FIG. 4 shows that a laminate comprising the carbonization catalyst film11, the graphene sheet 13 formed on a surface of the catalyst film 11,the binder layer 15 formed on a surface of the graphene sheet 13opposite catalyst film 11, and the substrate 17 disposed on a surface ofthe binder layer 15 opposite graphene sheet 13 may be immersed in anacid etching solution A for exfoliating the carbonization catalyst.Herein, though FIG. 4 shows the laminate of FIG. 3 immersed in acidsolution A, it will be understood that, in an alternate exemplaryembodiment, the laminate of FIG. 2 may be also immersed in the acidsolution A.

The carbonization catalyst film 11 may be, in the exemplary embodiment,exfoliated by the acid solution A. The acid of the acid solution may be,for example, a strong acid such as H₂SO₄, HNO₃, H₃PO₄, HCl, anycombination thereof, or the like. In an embodiment, where thecarbonization catalyst is Ni for example, the chemical process by whichcarbonization catalyst is exfoliated by the acid solution may be shownin the following Reaction Formula 1.

Ni(s)+2H⁺O+SO₄ ⁻[or H⁺+NO₃ ⁻, 3H⁺+PO₄ ³⁻, H⁺+Cl⁻, etc.]→NiSO₄[orNi(NO₃)₂, Ni₄(PO₄)₃, Ni(Cl)₂, etc.]+H₂(g)

Reaction Formula 1

Because the carbonization catalyst film 11 may have a large thickness aswell as a large area, the etching rate during the wet etching may bedifferent depending on the position on the carbonization catalyst film11 where the etch rate is measured. As a result, differences in etchingrate across the carbonization catalyst film 11 may result in damage tothe graphene sheet. Accordingly, it may be available to control theetching rate across the carbonization catalyst film 11 uniformly.

Referring again to FIG. 4, bubbles B [hydrogen, air, or other gas] andpits P in the surface of the carbonization catalyst film 11 may begenerated as the acid solution A reacts with the carbonization catalyst.This reaction may occur very vigorously. This may mean that it may bepossible to control the etching rate across the carbonization catalystfilm 11 uniformly through controlling the generation of the bubbles Band the pits P during the exfoliation reaction.

In an exemplary embodiment, a wetting agent may be used to control theetching rate across the carbonization catalyst film 11 uniformly and tofurther prevent damage of the graphene sheet since the wetting agent maybe used for controlling the generation of the bubbles B and the pits Pduring the exfoliation of the carbonization catalyst film 11. That is,the wetting agent may reduce the contact angle between the bubbles B andthe carbonization catalyst film 11 and help the bubbles B to be removedbefore they grow into a large size enough to generate the pits P in thesurface of the carbonization catalyst film 11.

FIG. 5 is a schematic view showing the effect of a wetting agent addedto the acid etching solution in FIG. 4.

Referring to FIG. 5, a wetting agent compound W may bond to thecarbonization catalyst film 11. Since the wetting agent compound asbonded to the surface of the carbonization catalyst film 11 in FIG. 5 ismerely presented in a simplified or exaggerated manner, it will beunderstood that the individual molecules of wetting agent are notrepresented in their actual size, shape or bonding state.

To prevent bubbling and pitting in the carbonization catalyst film 11,it may be available to use a linear carbon compound as the wetting agentW in preference to a branched carbon compound.

In an embodiment, a non-limiting examples of the wetting agent Winclude, but are not limited to, a sulfate, sulfonate, carboxylate,phosphate, nitrate, or the like. In an exemplary embodiment, a sulfonateor sulfate may be used.

In an embodiment, non-limiting examples of sulfonate and sulfate includea C₈-C₁₈ normal primary alcohol sulfate, a C₁₀-C₁₉ aromatic substitutedbenzene sulfonate, combinations thereof, and the like.

In an embodiment, non-limiting examples of sulfonate and sulfate includesodium lauryl sulfate, sodium monolaurin monosulfate, lauric acidmonoester of diethylene glycol sulfoacetate sodium salt,dodecyloxymethanesulfonate, any combinations thereof, and the like.

Use of insufficient amounts of the wetting agent may result in damage toa larger area of the graphene sheet during the exfoliation of thecarbonization catalyst. To prevent damage to the graphene sheet, it maybe available to include the content of the wetting agent in an amount ofabout 0.1 wt % to about 0.5 wt % of the total weight of the solution. Inan embodiment, effective prevention of the damage of the graphene sheet,may be provided by use of, for example including the wetting agent in anamount of about 0.3 wt %. Use of wetting agent in an amount greater than0.5 wt % may not result in any additional damage-preventing, despiteincreasing the amount of the wetting agent.

FIG. 6 is a schematic view of the graphene sheet of FIG. 5 afterremoving from the etching solution (i.e., after exfoliation).

FIG. 6 shows the graphene sheet 13, the binder layer 15 disposed on asurface of the graphene sheet 13, and the substrate 17 disposed on asurface of the binder layer 15 opposite the graphene sheet 13. As shownin FIG. 6, the carbonization catalyst 11 (not shown in FIG. 6) has beenexfoliated from the graphene sheet 13.

FIG. 7 is a schematic view of a process for transferring the graphenesheet of FIG. 6 from which the carbonization catalyst is exfoliated to adevice.

In FIG. 7, in an embodiment, a field-effect transistor 20 is illustratedin an exploded view to exemplify the device. The field-effect transistor20 may include an insulating layer 22 formed on a surface of back gate21. A drain electrode 25 and a source electrode 27 may each be formed ona surface of the insulating layer 22 opposite back gate 21. Infabricating the device, the graphene sheet 13 may be transferred to theregion between the drain electrode 25 and the source electrode 27 (shownby the arrow in FIG. 7).

Though FIG. 7 illustrates a process of transferring the graphene sheetto a device, it will be understood to the skilled in the art that thegraphene sheet 13 does need not necessarily be transferred. For example,in another embodiment, the graphene sheet 13 may be cut into a selectedshape or may be rolled to have a tube form for a specific use. Agraphene sheet so processed may also be used where coupled with adesired object.

As described above, when exfoliating a carbonization catalyst from agraphene sheet on which the carbonization catalyst has grown, thegraphene sheet may be obtained without damage and may be readilytransferred to a desired device by forming a binder layer which maysupport and fix the graphene sheet. Thus obtained graphene sheet mayhave a larger area than the graphene sheet obtained through otherphysical methods and may be useful in various applications includingtransparent electrodes, conducting thin films, hydrogen storage media,optical fibers, electronic devices, and the like. Further, since thusobtained graphene sheet may be not self-rolling even after it is exposedto the air, the graphene sheet may be useful in the variousapplications. Non-limiting exemplary ways evaluating the extent ofdamage to a graphene sheet includes using photographs of the damagedgraphene sheet. For example, after taking a photograph of the damagedgraphene sheet, if the graphene sheet is shown to have a damage such asa hole around the center of the graphene sheet in the photograph, thedamage extent may be evaluated from the ratio of the damaged area, i.e.,the hole area to the catalyst layer area (=the whole graphene sheet areabefore its damage). If a part of the graphene sheet is shown to be cutout in the photograph, the damage extent may be evaluated from the ratioof the damaged area i.e., the cut-out area to the catalyst layer area(=whole graphene sheet area before its damage). As for non-limitingexamples, the damaged area may be estimated by enlarging the damagedarea in the photograph and multiplying a total number of minute unitareas having a shape such as square (not limited to the square shape)filled without margin into the damaged area by the unit area. The shapeand/or size of the unit area may be varied considering the complexity ofthe shape of the damaged part.

The invention will now be described in further detail with reference tothe following examples. The following examples and experiments are forillustrative purposes only and not intended to limit the scope of theclaimed invention.

[Experiment 1: Variation in composition of binder layer]

An about 0.5 mm-thick nickel plate with a size of about 1.2 cm×about 1.5cm is prepared as a film-shaped carbonization catalyst. The nickel plateis positioned in a chamber, and heat-treated at about 1,000° C. forabout 5 minutes using a halogen lamp heater while supplying acetylenegas into the chamber at a constant rate of about 200 sccm in order toform graphene on the film-shaped carbonization catalyst (i.e., on thenickel plate).

Subsequently, after removing the heater, the inside of the chamber iscooled rapidly. Graphene is thus grown to have a uniform spatialarrangement, and in this way an about 10-layered graphene sheet with asize of about 1.2 cm×about 1.5 cm is obtained.

On the about 10-layered graphene sheet, a coating layer of a bindermaterial and a substrate (PET) are formed. The binder layers are formedto have a thickness of about 1 μm.

The coating of the binder material is carried out so that the bindermaterial cover the graphene sheet in whole or in part, on the surfaceopposite the nickel plate. In Example 1 through Example 6 of thisexperiment, the binder material is coated to cover the entire surface ofthe graphene sheet, on the surface opposite the nickel plate, and thencured.

In Example 7, the binder material is coated to overlap the edge of thenickel plate and the graphene sheet, in order for the binder material tosupport and fix the graphene sheet at its edge portion (about 0.1 cm inwidth around the periphery of the graphene sheet). In Example 7, thenickel plate is placed on a polyethylene terephthalate (PET) substratewith the graphene sheet oriented cofacially with the PET substrate sothat the PET substrate contacted the graphene sheet. Subsequently, thebinder material (e.g., Ag paste as described below) is applied at theedge portion of the nickel plate and the graphene sheet and then cured,which enables only the edge portion of the graphene sheet to besupported and fixed by the binder material. It may be necessary to takecare in applying the binder to prevent the binder material from enteringthe region of contacting surfaces between the graphene sheet and thesubstrate. Once the nickel plate is exfoliated, the corresponding edgeportion may be cut off so as not to be used afterward together with thegraphene sheet.

For the binder material, a siloxane-based compound (PDMS, SYLGARD® 184,Dow Coming) [Example 1], an acryl-based compound (PMMA; poly(methylmethacrylate), Mw=about 2,480,000, FLUKA) [Example 2], an epoxy resin(EPICON® HP-820, DIC) [Example 3], a conductive polymer(poly(3-hexythiophene), regioregular P3HT, Aldrich) [Example 4], apolymer electrolyte material (poly(sodium 4-styrenesulfonate), Aldrich)[Example 5], a photoresist material (AZ-111 photoresist, MicroChem)[Example 6] and an Ag paste (Fujikura Kasei. Co. Ltd.) [Example 7] areused. In Comparative Example, a binder layer is not formed.Subsequently, the laminate comprising the carbonization catalyst, thegraphene sheet and the binder layer is immersed in an about 30% (v/v)aqueous HNO₃ solution for about 24 hours to exfoliate the nickelcatalyst. In all Examples and Comparative Example, about 0.3 wt % (sat.)potassium perfluorooctanesulfonate is used as a wetting agent.

Damage of the binder layer due to the acid, curability, dissociabilityand transparency are evaluated as the examples and comparative example.

As described above, the damage of the binder layer due to the acid maybe represented as a percentage ratio of the increased etching time tothe time required for etching in the case that the binder is not used.Further, as described, if the damage of the binder layer is about 0%, itmay further mean that there may be no discoloration of the graphenesheet caused by over etching, where the etching time is the same as whenthe binder layer is not used.

Curability and dissociability are evaluated as follows.

Curability is evaluated by visual observation with the unaided eye, whenanother film (for example, PET film) is attached to the surface on whichthe binder is coated and then detached therefrom, to determine whetherthe binder is adhered to the film. Dissociability is evaluated byobserving visually whether the binder dissolved in acetone.

Transparency is evaluated by measuring light transmittance using aspectrophotometer.

The experiment result is given in the following Table 1.

TABLE 1 Damage to binder layer Curability¹ Dissociability² TransparencyExample 1 About 0% ◯ X About 100% Example 2 About 0% ◯ ◯ About 95%(Transparency may be varied according to thickness and may have a valuegreater than about 90%) Example 3 About 10% Δ X About 85% (Transparencymay be varied according to thickness and may have a value greater thanabout 80%) Example 4 About 10% X X About 50% (Transparency may be variedaccording to thickness and may have a value less than about 50%) Example5 About 80% ◯ X About 70% (Transparency may be varied according tothickness and may have a value greater than about 70%) Example 6 About10% ◯ ◯ Binder layer removed Example 7 About 10% ◯ X Irrelevant (Binderis used only at the edge portion) ¹◯: not adhered to the film, Δ: partlyadhered to the film, X: adhered to the film. ²◯: dissolved in acetone,X: not dissolved in acetone.

Damage to the graphene sheet after transferring to a device isevaluated. After exfoliation of the nickel plate, the relative area ofthe damaged graphene sheet is estimated as a percentage ratio of thedamaged area to the total area of the graphene sheet area (i.e. the areaof the nickel plate). The result is given in the following Table 2.

TABLE 2 Example 1 About 2% Example 2 About 5% Example 3 About 5% Example4 About 25% Example 5 About 30% Example 6 About 20% Example 7 About 5%Comparative Example About 60%

As seen in Table 2, the damage to the graphene sheet is prevented whenthe binder layer is used (Examples) as compared with ComparativeExample. The damage to the graphene sheet is smaller when a binder layerwith less damage due to acid (for example, about 10%, or about 0%) isused.

Meanwhile, when only the edge portion is fixed (Example 7), the damageof the graphene sheet is also small.

In the Comparative Example, rolling of the graphene sheet is observedwhen the graphene sheet is exposed to air after removal from the etchantsolution. Further, damage (tearing) takes place where there is avibration or contact with the substrate.

[Experiment 2: Inclusion of wetting agent]

A graphene sheet is obtained in the same manner of Example 1 ofExperiment 1, except for the differences in the wet etching step asnoted hereinbelow.

As etchant solution for wet etching, about 55 ml, about 40 ml and about25 ml of about 60 wt % nitric acid (HNO₃) solutions are prepared.Potassium perfluorooctanesulfonate (wetting agent) of the followingChemical Formula 1 is added to nitric acid (sat.) to prepare about 15ml, about 30 ml and about 55 ml of etching solutions having about 0.5 wt% wetting agent.

The volume of water is fixed at about 45 ml, and the volume of thewetting agent solution and the about 60 wt % nitric acid solution iscontrolled as in the following Table 3 in order to control theconcentration of the wetting agent in the entire etchant solution.

TABLE 3 Example 1 Example 2 Example 3 Example 4 Nitric acid solution(about 60 wt %) — About 25 ml About 40 ml About 55 ml Wetting agentsolution [about 0.5 About 55 ml About 30 ml About 15 ml — wt % wettingagent in nitric acid (sat.)] Water About 45 ml About 45 ml About 45 mlAbout 45 ml Concentration of nitric acid in the About 33 wt % About 33wt % About 33 wt % About 33 wt % etching solution Concentration ofwetting agent About 0.275 wt % About 0.15 wt % About 0.075 wt % About 0wt %

Damage of the graphene sheet is evaluated for Examples 1 to 4 (measured1 day after beginning etching). As in Experiment 1, after exfoliation ofthe nickel plate, the area of the damaged graphene sheet is calculatedas percentage ratio of the damaged part to the graphene sheet area (i.e.area of the nickel plate). The result is given in the following Table 4.

TABLE 4 Example 1 About 5% Example 2 About 25% Example 3 About 50%Example 4 About 60%

As seen in Table 4, where the concentration of the wetting agent waslower (Example 3), there is more damage to the graphene sheet during theexfoliation of the catalyst. When no wetting agent is used (Example 4)the damage reached about 60% or more beyond about 50%. Accordingly, itmay be said that the damage of the graphene sheet may be further reducedby increasing the amount of the wetting agent.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madethereto without departing from the spirit and scope of the invention asdefined by the appended claims.

In addition, modifications can be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method for exfoliating a carbonization catalyst from a graphenesheet comprising: forming a binder layer on a surface of the graphenesheet, wherein the binder layer supports and fixes all or part of thegraphene sheet formed on the carbonization catalyst; and exfoliating thecarbonization catalyst from the graphene sheet.
 2. The method accordingto claim 1, wherein exfoliating comprises reacting an etch solution ofan acid with the carbonization catalyst to exfoliate the carbonizationcatalyst.
 3. The method according to claim 2, wherein a wetting agent isused together with the acid in the etch solution.
 4. The methodaccording to claim 3, wherein the amount of the wetting agent is about0.01 to about 0.5 wt % based on the total weight of the etch solutionincreased to reduce damage of the graphene sheet.
 5. The methodaccording to claim 2, wherein damage to the binder layer due to the acidextends to about 0% to about 50% of the thickness of the binder layer.6. The method according to claim 1, wherein the binder layer comprises amaterial having at least one property selected from the group consistingof curability, dissociability and transparency.
 7. The method accordingto claim 1, wherein the binder layer is formed of at least one binderselected from the group consisting of a siloxane-based compound, anacryl-based compound, an epoxy-based compound, a conductive polymer, apolymer electrolyte material, a photoresist (PR) material, a metal pasteand any combinations thereof.
 8. The method according to claim 1,wherein a substrate is further disposed on a surface of the binder layeropposite the graphene sheet before or after the binder layer is formedon the surface of the graphene sheet.
 9. The method according to claim1, wherein the binder layer supports and fixes only the edge portion ofthe graphene sheet.
 10. The method according to claim 1, wherein afterthe exfoliation of the carbonization catalyst, the binder layer isremoved from the graphene sheet.
 11. The method according to claim 1,wherein the carbonization catalyst is at least one catalyst selectedfrom the group consisting of Ni, Co, Fe, Pt, Au, Al, Cr, Cu, Mg, Mn, Mo,Rh, Si, Ta, Ti, W, U, V, Zr, and any combinations thereof.
 12. A methodfor transferring a graphene sheet from which a carbonization catalyst isexfoliated to a device comprising: forming a binder layer on a surfaceof a graphene sheet formed on a surface of a carbonization catalystwhere the binder layer and carbonization catalysts are on oppositesurfaces of the graphene sheet, wherein the binder layer supports andfixes all or part of the graphene sheet; exfoliating the carbonizationcatalyst from the graphene sheet; and transferring the graphene sheetfrom which the carbonization catalyst is exfoliated to a device.
 13. Themethod according to claim 12, where the binder layer is removed from thegraphene sheet.
 14. The method according to claim 12, whereinexfoliating the carbonization catalyst is accomplished with an acid anda wetting agent.
 15. The method according to claim 12, wherein asubstrate is further formed on a surface of the binder layer oppositethe graphene sheet before or after the binder layer is formed on thesurface of the graphene sheet.
 16. A graphene sheet obtained by forminga binder layer on a surface of a graphene sheet which is formed on asurface of a carbonization catalyst, where the binder layer andcarbonization catalyst are on opposite surfaces of the graphene sheetand the binder layer supports and fixes all or part of the graphenesheet formed on the carbonization catalyst, and exfoliating thecarbonization catalyst from the graphene sheet.
 17. The method accordingto claim 16, where the binder layer is removed from the graphene sheet.18. The graphene sheet according to claim 16, wherein exfoliating iscarried out with an etching solution comprising an acid and a wettingagent.
 19. The graphene sheet according to claim 16, wherein a substrateis further formed on a surface of the graphene sheet opposite thecarbonization catalyst.
 20. The graphene sheet according to claim 16,wherein the graphene sheet is formed of polycyclic aromatic moleculeswith carbon atoms covalently bonded and comprises about 1 to about 300layers in thickness direction, and is greater than or equal to about 1mm along each of the transverse and longitudinal directions.
 21. Thegraphene sheet according to claim 16, wherein the graphene sheet is notrolled upon exposure to air.
 22. The graphene sheet according to claim16, wherein a damaged area of the graphene sheet after exfoliating isless than or equal to about 50% of the total area.
 23. A method offorming a device, comprising: forming a graphene sheet on a surface of acarbonization catalyst, forming a binder layer on a surface of thegraphene sheet opposite the carbonization catalyst, wherein the binderlayer supports and fixes all or part of the graphene sheet formed on thecarbonization catalyst, exfoliating the carbonization catalyst from thegraphene sheet, and transferring the graphene sheet to the device.
 24. Adevice formed by the method of claim 23.